Process for the vulcanisation of natural and/or synthetic rubbers made from halogen-free dienes

ABSTRACT

The invention relates to a vulcanisation accelerator combination and their use for a process for vulcanising natural and/or synthetic rubber made from halogen-free dienes. As vulcanisation accelerators triazines substituted in the 2, 4 and 6 positions in combination with at least one thiazole accelerator and optionally at least one thiuram accelerator are used. This accelerator combination obviate the disadvantage of the long cure time when using the triazine accelerator alone without loss of the advantage of the high moduli.

States Patent Kemperann et a1.

[ 1 Oct. 29, 1974 PRGCESS FOR THE VULCANISATION OF NATURAL AND/0R SYNTHETIC RUBBERS MADE FROM HALOGEN-FREE DIENES Inventors: Theo Kempermann; Ulrich Eholzer,

both of Cologne; Hermann Westlinning, Kleinostheim; Horst Fleischhauer, Grossauheim, all of Germany Assignees: Bayer Aktiengesellschalt,

Leverkusen; Deutsche Gold-Und Silber-Scheideanstalt Vormals Roessler, Frankfurt/Main, both of, Germany Filed: Apr. 24, 1972 Appl. No.: 246,646

Foreign Application Priority Data Apr. 26, 1971 Germany 2120288 11.5. C1 252/182, 260/458 NT, 260/795 B, 260/249.5, 260/784, 260/791 Int. Cl. C08f 45/48, C08g 51/48 Field of Search 252/182; 260/249.5, 784, 260/795 B, 45.8 NT, 791

[56] References Cited UNITED STATES PATENTS 3,240,749 3/1966 Dexter et al 260/2495 3,250,772 5/1966 Dexter et a1. 260/2495 3,413,268 11/1968 Feichtinger et a1... 260/249 5 3,415,824 12/1968 Biland et a1 260/249 5 3,558,739 l/1971 Kagarise 260/784 Primary ExaminerCarl D. Quarforth Assistant Examinerlrwin Gluck Attorney, Agent, or FirmC0nno11y and Hutz [5 7 ABSTRACT 8 Claims, No Drawings PROCESS FOR THE VULCANISATION OF NATURAL AND/OR SYNTHETIC RUBBERS MADE FROM HALOGEN-FREE DIENES This invention relates to a process for the vulcanisation of natural and synthetic rubbers which have been made from dienes which are halogen-free in the presence of sulphur and/or sulphur donors and in particular to vulcanisation accelerators for such a process.

It is known to use derivatives of 1,3,5-triazine of the general formula (1) as vulcanisation accelerators for the sulphur vulcanisation of diene rubbers (British Patent Specification No. 1,095,219).

if c Y -SJJ In general formula (I),

X represents hydrogen, alkyl, substituted alkyl, alkenyl. substituted alkenyl, aryl, substituted aryl, aralkyl, substituted aralkyl radicals or a heteroatom which is attached to one or more of the abovementioned substituents,

Y and Z may be the same or different and represent a. hydrogen, b. the radical -SR wherein R denotes an alkyl, alkenyl, aryl or aralkyl radical or a heterocyclic ring system, c. the radical wherein R and R are the same or different and represent hydrogen, alkyl, alkenyl, aryl or aralkyl groups, or

d. a radical of the general formula (ll) wherein A may denote cm. 0, s, or NII.

in which the H may be substituted.

A vulcanisation accelerator which is particularly suitable for the sulphur vulcanisation of diene rubbers is 2-diethylamine-4.6-bis-(cyclohexyl-sulphenamido)-striazine. hereinafter referred to as triazine accelerator B. which results in high moduli of the vulcanisates even when used in small doses.

The derivatives of 1,3,5-triazine of the general formula (l), and especially accelerator B have, however,

one disadvantage when used alone as accelerators. Be-

cause they only cause a slow vulcanisation, they require a relatively long cure time. Their flow time/cure time ratio is therefore unfavourable.

5 ltis "also known that mercapto accelerators (see S. Bostrom, Kautschuk-Handbuch, volume 4, pages 300'307, et seq., Stuttgart 1961) such as dibenzothiazyl disulphide (MBTS), 2-mercaptobenzothiazole (MBT) or the zinc salt of 2-mercaptobenz0thiazole give rise to relatively low moduli when used as sole accelerators, especially if only low doses of sulphur are used (about 0.6 to 1.5 phr of sulphur) as is the case with so-called efficient or semi-efficient vulcanisation systems used for the production of heat-resistant vulcanisates. For comparison reference is made to Example 2 (mixture No. 7) in which dibenzothiazyl disulphide (MBTS) gives rise to a relatively low modulus when the sulphur dose is 1.0 phr. The above-mentioned accelerators of the mercapto accelerator type do not give satisfactory yields and must therefore be used in relatively high doses, especially when only small quantities of sulphur are used.

The use of sulphenamides of 2-mercaptobenzothiazole is also known (see Ullmanns Enzylklopadie der technischen Chemie, 3rd edition, volume 9, page 386, publishers Urban & Schwarzenberg, Munich Berlin 1957). As can be seen from Example 1, N- cyclohexyl benzothiazole-2-sulphenamide (CBS), which is the most frequently used of these sulphenamides in practice, is much less effective'than the triazine accelerator B. A dose of only 0.5 phr of triazine accelerator B provides practically the same maximum modulus as twice that dose of CBS (1.0 phr).

The invention is based on the problem of finding a vulcanisation accelerator which would obviate the disadvantage of the long cure time without loss of the advantage of the high moduli.

The problem was solved by finding a mixture of special accelerators for vulcanisation.

This invention therefore relates to a process for the vulcanisation of natural and synthetic rubbers made from halogen-free dienes in the presence of sulphur and/or sulphur donors and vulcanisation accelerators, wherein the vulcanisation accelerators used are combinations of a. 2,4,6-triazine compounds of the general formula in which the radicals R, and R may be the same or different and represent hydrogen, straight-chain or branched chain alkyl groups containing one to six carbon atoms or phenyl groups or R and R taken together may form a 5- to 7-membered ring which may contain an additional oxygen atom.

R and R may be the same or different and represent straight-chain or branched chain alkyl radicals containing one to six carbon atoms or taken together may form a 5- to 7-membered ring which may also contain an additional oxygen atom, and one of the radicals R or R may be hydrogen; b. a vulcanisation accelerator of the thiazole type and optionally c. a vulcanisation accelerator of the thiuram type. This invention also relates to mixtures of the vulcanisation accelerators and mixtures of the accelerators with natural and synthetic rubbers produced from halogen-free dienes.

The vulcanisation accelerator preferably used is a mixture of 2-diethylamino-4,6-bis-(cyclohexyl-sulphenamido)-s-triazine (see group a), dibenzothiazyl disulphide or N-cyclohexyl-benzothiazole-2- sulphenamide (see group b) and optionally tetramethylthiuram disulphide or tetramethyl thiuram monosulphide (see group 0).

The following are mentioned as further examples of suitable compounds of group a of the general formula (Ill): 2-methyleno-4,6-bis-( N-cyclohexyl-sulphenamido )-striazine; 2-ethylamino-4.6-bis-( N-c yclohexyl-sulphenamido )-striazine; 2-n-propylamino-4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine; 2-iso-propylamino-4,6-bis-(N- cyclohexyl-sulphenamido)-s-triazine; Z-n-butylamino- 4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine; 2-isobutylamino-4,6-bis-(N-cyclohexyl sulphenamido)-striazine; 2-dimethylamino-4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine; 2-di-n-propylamino-4,6-bis-(N- cyclohexyl-sulphenamido)-s-triazine; 2-di-iso-propylamino-4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine; V Zdi-n-butylamino-4,6-bis-( N-cyclohexyl-sulphenamido)-s-triazine; 2-di-is0-butylamino-4,6-bis-( N-cycloheXyl-sulphenamido)-s-triazine; 2-methylamino-4,6-bis-(N-tert.-butyl-sulphenamido)- s-triazine; 2-ethylamino-4,6-bis-(N-tert.-butyl-sulphenamido)-striazine; 2-n-propylamino-4,6-bis-(N-tert.-butyl-sulphenamido )-s-triazine; 2-iso-propylamino-4,6-bis-( N-tert.-butyl-sulphenamido)-s-triazine; 2-n-butylamino-4,6-bis-(N-tert.-butyl-sulphenamido)- s-triazine; 2-iso-butylamino-4,6-bis-( N-tert.-butyl-sulphenamido)-s-triazine; 2-dimethylamino-4,6-bis-( N-tert.-butyl-sulphenamido)-s-triazine; n 2-dimethylamino-4,6!bis-( N-tert.-butyl-sulphenamido)-s-triazine; 2-di-n-propylamino-4,6-bis-( N-tert.-butyl-sulphenamido)-s-triazine; 2-di-propylamino-4,6-bis-( N-tert.-butyl-sulphenamido )-s-triazine; 2-di n-butylamino-4;6-bis-(N-tert.-butyl-sulphenamido)-s-triazine; 2-di-iso-butylamino-4,6-bis-( N-tert.-butyl-sulphenamido)-s-triazine; 2-methylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-ethylamino-4,6-bis-( N-morpholinyl-sulphenamido s-triazine;

4 Z-n-propylamino-4,6-bis-( N-morpholinyl-sulphenamido)-s-triazine; 2-iso-propylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-n-butylamino-4,6-bis-( N-morpholinyl-sulphenamido)-s-triazine; 2-iso-butylamino-4,6-bis-( N-morpholinyl-sulphenamido)-s-triazine; 2-dimethylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-diethylamino-4.6-bis-( N-morpholinyl-sulphenamido)-s-triazine; 2-di-n-propylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-di-iso-propylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-di-n-butylamino-4,6-bis-(N-morpholinyl sulphenamido)-s-triazine; 2-di-iso-butylamino-4,6-bis-(N-morpholinyl-sulphenamido)-s-triazine; 2-tert.-butylamino-4,6-bis-(N-tert.-butyl-sulphenamido)-s-triazine; 7 2-amino-4,6-bis-(N-cyclohexyl-sulphenamido)-striazine; 2-morpholino-4,6-bis-(N-piperidino-suIphenamido)-striazine; 2-anilido-4,6-bis-(N-cyclohexyl-sulphenamido)-striazine; 2-diphenylamino-4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine.

The following are given as examples of vulcanisation accelerators from the group of thiazole accelerators (group b): N-cyclohexyl-2-benzothiazole-sulphenamide (CBS); dibenzothiazyl disulphide (MBTS); 2-mercaptobenzothiazole (MBT); the zinc salt of Z-mercaptobenzothiazole; N-tert.-butyl-2-benzothiazole-sulphenamide (TBS) and 4-(benzothiazole-2-sulphenyl)-morpholine (OBS). N-cyclohexyl-2-benzothiazole-sulphenamide and dibenzothiazole disulphide are preferred.

The following are given as examples of vulcanisation accelerators from the group of thiuramic accelerators (group c) which may be included if desired: tetraethylthiuram disulphide (TETD) tetramethylthiuram monosulphide (TMTM), dimethyl diphenylthiuram disulphide and tetramethylthiuram disulphide (TMTD). Tetramethylthiuram disulphide and tetramethylthiuram monosulphide are preferred.

The process according to the invention succeeds in obviating the disadvantage of the long cure time required for hitherto known triazine accelerators without any loss of their main advantage i.e. the high moduli which can be obtainned by their use even at low dosages. The combination according to the invention therefore combine the advantages of short cure times at low doses with resulting high moduli and therefore operate very economically. It could not be foreseen that the high degree of cross-linking of the vulcanisates would be preserved when replacing part of the triazine accelerator, which is very effective when used alone, with a thiazole accelerator which is much less effective when used alone.-

The individual components a, b and c of the accelerator combinations according to the invention may advantageously be used in the following quantities (parts by weight based on 100.0 parts by weight of rubber):

triazine accelerator from about 0,05 to about 3, preferably 0.1 to 0.5;

thiazole accelerator from about 0.1 to 4, preferably Thiuram accelerator from 0 to 2, preferably 0.01 to The quantity of sulphur to be used is between 0.2 and about 4 parts by weight. Sulphur donors such as N,N-dithio-bis-morpholine, dipentamethylenethiuram tetrasulphide, N,NZdithio-bis-hexahydro-ZH- azcpinone-(Z) and 2-benzothiazyldithio-N-morpholine may also be used.

The process according to the invention is very useful for vulcanising diene rubber which have been made from dienes which are halogen-free. The following diene rubbers are given as examples: styrene-butadiene rubber (SBR), natural rubber (NR), nitrile rubber (NBR), polybutadiene (BR), polyisoprene (1R), transpolypentenamer (trans-PA) (see Kautschuk und Gummi, Kunststoffe 23, 502 et seg. 1970) publishers: Verlag fiir Radio, Photo and Kimtechnik, Berlin). Styrene-butadiene rubbers are preferred.

There may also be used mixtures of various diene rubbers.

The individual components of the accelerator system may be added to the dine rubbers or rubber mixtures before vilcanisation, either separately or in the form of a mixture or a rubber-accelerator master batch (see Ullmanns) Enzyklopiidie der technischen Chemie, 3rd edition, publishers: Urban & Schwarzenberg, Munich Berlin 1957, volume 9, page 364).

The diene rubbers may contain one or more ofthe usual additives such as fillers, in particular carbon black, mineral oils, plasticisers, adhesifying agents, accelerator activators, especially stearic acid and/or zinc oxide, waxes, age-resistors, anti-ozone agents, blowing agents. dyes and pigments.

Fillers such as the various types of carbon black used in the rubber processing industry, silica, particularly in a finely divided form obtained by wet precipitation or in the gaseous phase from volatile silicon halides and hydrophobic silica, and finely divided metal oxides including mixed oxides and oxide mixtures are valuable constituents for the mixture.

vulcanisation of the diene rubbers is generally carried out at temperatures of between about 100 C. and about 300 C., preferably at 140 C. to 240 C. Any of the usual vulcanisation processes used in the art as heating in a press or heating with super-heated steam or hot air or in a salt bath or fluidised bed or with ultrahigh frequency or in a steam pipe may be used.

The effectiveness of one of the accelerator combinations according to the invention in styrene-butadiene rubber is illustrated in Example 1. Triazine accelerator B (mixture 2) requires a long cure time (r Dibenzothiazyl disulphide (MBTS) used alone (mixture 4) in the same dose (0.5 phr) gives rise to a slightly lower modulus and also requires a relatively long total cure time.

The combination according to the invention of 0.25 phr of triazine accelerator B and 0.25 phr of MBTS (mixture 3) surprisingly requires a shorter total cure time than MBTS used alone and above all requires only about 67 percent of the total cure time of B used alone. The modulus obtained with the combination is distinctly higher than that obtained with either of the two individual components used in the same total dose.

Example 1 illustrates the high yield of triazine accelerator B which when used in a dose of only 0.5 phr (mixture 2) provides practically the same modulus maximum as 1.0 phr of N-cyclohexylbenzothiazole-2- sulphenamide (CBS) (mixture 1) but requires more than twice the total cure time Accelerator combinations frequently used in practice for the sulphur vulcanisation of diene rubbers consist of mercapto accelerators such as Z-mercaptobenzothiazole (MBT) or dibenzothiazyl disulphide (MBTS) and diphenyl guanidine (DPG) (see S. Bostrom, Kautschuk-Handbuch, volume 4, Stuttgart 1961, pages 300-304 and pages 320-321).

Example 1 also demonstrates the superiority of a combination according to the invention of triazine accelerator B and dibenzothiazyl disulphide, which in this case also contains tetramethylthiuram disulphide as additional accelerator (mixture 5), over a conventional combination of dibenzothiazyl disulphide and diphenyl guanidine (mixture 6). The accelerator combination according to the invention (mixture 5) consists of a total of only 0.3 phr of accelerator compared with 1.0 phr of accelerator in mixture 6 and yet is superior in essential features and provides better resistance in scorching and higher modulus maximum and requires practically the same total cure time.

Example 2 demonstrates the effectiveness of a combination according to the invention of dibenzothiazyl disulphide (MBTS) and triazine accelerator B (mixture 8) compared with that of triazine accelerator B used alone (mixture 9), the total dose being the same in both cases and used with a low sulphur dose (1.0 phr) of the I kind which may be used for the production of heatresistant vulcanisates.

With this low sulphur dose, dibenzothiazyl disulphide (MBTS) (mixture 7) results in only a relatively low modulus. The modulus obtained with the combination according to the invention (mixture 8) is substantially higher than that obtained with MBTS and even higher than that obtained with the not combined triazine accelerator B (mixture 9). The total cure time is slightly shorter than that required for the not combined products.

Example 3 demonstrates the effectiveness of the accelerator combination according to the invention when used in natural rubber. Dibenzothiazyl disulphide (MBTS) used along (mixture 10) provides a relatively low modulus. Mixtures ll, 12 and 13 in which combinations of MBTS and triazine accelerator B were used as vulcanisation accelerators result in moduli which are significantly higher than those obtained with mixture 10 and in part also higher than those obtained with mixture 14 which contains triazine accelerator B on its own. Above all, mixtures 1 l, 12 and 13 require a shorter total cure time than mixtures 10 and 14 with the uncombined individual components.

Example 4 shows the results obtained with a combination of triazine accelerator B and N- cyclohexylbenzothiazole-2-sulphenamide (CBS). CBS

used alone (mixture 15) results in a substantially lower modulus than the mere triazine accelerator B (mixture 17). The combination according to the invention (mixture 16) of the two accelerators results in practically the same modulus as the mere triazine accelerator but with a shorter total cure time.

A combination with even better yield is obtained by adding a very small quantity of tetramethylthiuram monosulphide (TMTM) (mixture 18). This combina- Table 1 -Continued Test formulations i 11 tion, which contains a total of only 0. 47 phr of acceler- Constituents of the (Example (Example 3) ators. provides the same scorch resistance and the same mixture (phr) 1L4) modulus and requires the same total cure time as l.0 H, h h f k k 45 0 phr of CBS (mixture This combination according i b fi fi', T to the invention of triazine accelerator B, thiazole ac- Carbonblack N 220. lSAF 42.0 celerator N-cyclohexylbenzothiazole-Z-sulphenamide i' zf gg 0 2:8 (CBS) and thiuram accelerator tetramethylthiuram 10 Naphthenic mineral oil 40 monosulphide (TMTM) is more economical than N- Phenyl-B-naphthylamme 0.5 L0 cyclohexyl-benzothiazole-2 -sulphe namide (CBS) by N-phenyl-N iso propyl-p- 1.5 L5 virtue of its surprisingly higher yield; it constitutes a P y dwmme f d b d t fth fth t. S l h r 4 as indicated pre erre em 0 men 0 e process 0 e inven ion. Accelerator m the exumpies l5 The data given in the following examples were obtained as follows: the test formulations listed in Table l were used to pre- T bl 2 pare rubber mixtures in an interval mixer by the usual 3 8 method. Sulphur and vulcanisation accelerator were v then mixed in by use ofa rolling mill. The mixtures then ubhrev'amns were U0 lfllO {BSI slabs of: 4 mm In thlckness y I) Scorch time By anulogBwith thc Mooney scorch vulcanisation in a press (stepwise heating at the tem- P f" 5 5 524) determined peratures indicated in the examples). The individual 232 data were obtained by the test methods indicated in ("30071 elongation by 20 points Table 2 above the minimum (step-cure) 2) M 300. M 400 Maximum modulus at 30071 elonga- Tlble 1 tion and 4009? elongation res ectively(kgflcm). DlN 53 504. Test formulations 3 s cm 2 i ll t V I 3) Total cure time Time required to reach 90% of (.QnhllitlLllLh of the tElxt nlplt (Example 3) [m (min) [ha maximum modulus (p r) at 3009i and 400% elongation.

respectively. at the tempera- Styrene-hutudiene ruhher [00.0 I Natural ruhhcr (smoked sheets) I000 mdlcned' Example 1 Styrene-butadiene rubber. 1.8 phr of sulphur (test formulation 1. see Table l Mixture Vulcanisation Dose Scorch time" M 300* Total cure time t No. accelerator (phr) at IC (min) Maximum modulus at l50C (min) at I50C (kgf/cm*) 1 CBS L0 99 24 2 B 0.5 43 98 54 3 {B 0.25

MBTS 0.25 38 l 13 36 4 MBTS 0.5 35 95 45 5 B 0.1

{MBTS 0. l 23 94 24 TMTD 0.1 6 M BTS 0.6

DPG 0.4 20 88 23 Exam le 2 tyrene-butadiene rubber. 1.0 phr of sulphur (test formulation I. see Table l Mixture Vulcanisation Dose Scorch time" M400" Total cure time t No. accelerator (phr) at l30C (min) Maximum modulus at IC (min) at I50C (Kgflcm i 7 MBTS 0.5 45 4| 8 {B 0.25 43 I I2 38 MBTS 0.25 9 B 0.5 53 I04 40 Example 3 Natural rubber, 2.35 phr of sulphur (test formulation 11, see Table 1 Mixture Vulcanisation Dose Scorch time M 300 Total cure time t No. accelerator Maximum modulus (phr) at 120C (min) at 150C at 150C (min) (kgf/cm 10 MBTS 0.5 18 88 12 11 {B 0.125 98 10.5

MBTS 0.375 12 B 0.25 105 10 MBTS 0.25 13 B 0.375 108 10.5

MBTS 0.125 14 B 0.5 53 102 16 Exam le 4 styrene-butadieae rubber, 1.8 phr of sulphur test formulation 1, see Table 1 Mixture Vulcanisation Dose Scorch time" M 300 Total cure time t,,,,"

No. accelerator Maximum modulus (phr) at 130C (min) at 150C at 150C (min) (Kgf/cm 15 CBS 1 .0 96 24 16 {B 0.5 53 120 28 CBS 0.5 17 B 1.0 55 121 36 18 B 0.2 40 97 24 CBS 0.2 TMTM 0.07

What we claim is: 30 2. The composition of claim 1 containing (c) up to 1. A vulcanization accelerator composition comprising a. 0.05 to about 3 parts by weight of at least one 2,4.6-triazine of the formula wherein R, and R which may be the same or different, are hydrogen, alkyl having one to six carbon atoms or phenyl, or R, and R taken together, complete with the nitrogen atom to which they are attached, a 5- or 7- membered ring which may contain oxygen as a hetero atom and R and R which may be the same or different, are each alkyl having one to six carbon atoms or one is alkyl having one to six carbon atoms and the other is hydrogen or R and R taken together, complete with the nitrogen atom to which they are attached, a 5- to 7-membered ring which may contain oxygen as a hetero atom and b. 0.1 to about 4 parts by weight of at least one thiazole accelerator.

2 parts by weight of at least one thiuram accelerator.

3. The composition of claim 1 wherein (b) is dibenzothiazyl disulphide, Z-mercaptobenzothiazole, the zinc salt of Z-mercaptobenzothiazole, N-cyclohexylbenzothiazole-Z-Sulphenamide, N-t-butylbenzothiazole-Z-sulphenamide or 4-(benzothiazole-2- sulphenyl)-morpholine.

4. The composition of claim 2 wherein (c) is tetrame-,

triazine, (b) is N-cyclohexyl-benzothiazole-2- sulphenamide and (c) is tetramethylthiuram monosulphide. 

1. A VULCANIZATION ACCELERATOR COMPOSITION COMPRISING A. 0.05 TO ABOUT 3 PARTS BY WEIGHT OF AT LEAST ONE 2,4,5TRIAZINE OF THE FORMULA
 2. The composition of claim 1 containing (c) up to 2 parts by weight of at least one thiuram accelerator.
 3. The composition of claim 1 wherein (b) is dibenzothiazyl disulphide, 2-mercaptobenzothiazole, the zinc salt of 2-mercaptobenzothiazole, N-cyclohexyl-benzothiazole-2-sulphenamide, N-t-butyl-benzothiazole-2-sulphenamide or 4-(benzothiazole-2-sulphenyl)-morpholine.
 4. The composition of claim 2 wherein (c) is tetramethylthiuram disulphide, tetraethylthiuram disulphide, tetramethylthiuram monosulphide or dimethyl diphenyl thiuram disulphide.
 5. The composition of claim 1 wherein (a) is 2-diethylamino-4,6-bis-(cyclohexyl-sulphenamido)-s-triazine and (b) is dibenzothiazole disulphide.
 6. The composition of claim 2 wherein (a) is 2-diethylamino-4,6-bis-(cyclohexyl-sulphenamido)-s-triazine, (b) is dibenzothiazole disulphide and (c) is tetramethylthiuram disulphide.
 7. The composition of claim 1 wherein (a) is 2-diethylamino-4,5-bis-(cyclohexyl-sulphenamido)-s-triazine and (b) is N-cyclohexyl-benzothiazole-2-sulphenamide.
 8. The composition of claim 2 wherein (a) is 2-diethylamino-4,5-bis-(cyclohexyl-sulphenamido)-s-triazine, (b) is N-cyclohexyl-benzothiazole-2-sulphenamide and (c) is tetramethylthiuram monosulphide. 